This application is a 371 of PCT/6898/02255 filed Aug. 6, 1998.
This invention relates to an improved fabric seam for paper making machine fabrics, and in particular to dryer fabrics for use in the dryer sections of paper machines.
In a typical dryer section, a dryer fabric carries a paper web in contact with one surface over a plurality of heated dryer rolls, with the paper web in contact with the dryer rolls surfaces, and over a plurality of unheated guide and drive rolls, with the non-paper carrying surface of the fabric in contact with the guide roll surfaces, the drying and guide rollers are typically arranged in a festoon of alternating rollers disposed in two lines, with the fabric carrying the paper web following a zig zag course about alternate drying and guide rolls.
It has been found in practice that because the paper web is on the inner face of the fabric about the drying rolls and on the outer face of the fabric about the guide/drive rolls, it has a different speed as it passes respectively about the drying rolls and the guide rolls and furthermore has a quickly alternating positive and negative speed differential with respect to the fabric and this results in friction between the paper web and the fabric. This causes a deterioration in the quality of the paper surface and an increase in wear on the paper contacting side of the fabric.
There exists a plane in the fabric which has a constant velocity throughout the path of the dryer belt through the machine. This is referred to as the neutral plane.
The position of the neutral plane has been shown to be linked to fabric symmetry and that by increasing the asymmetry of the fabric the neutral plane can be brought closer to the fabric face. It is desirable that the neutral plane should be as close as possible to the interface between the fabric and the paper web, to reduce the friction occurring between the paper and the fabric to a minimum. EP-A-0557572 discusses the theory of the neutral plane and discloses asymmetric fabric structures made of layers of differing thickness and differing modules of elasticity. Another fabric construction to displace the neutral plane is to construct the fabric from unequal yarns, e.g. weft or cross-machine direction yarns may be provided in two or more layers, with the thicker yarns forming a layer towards the non-paper carrying face of the fabric and a layer of thinner yarns being toward the paper contacting face of the fabric. The neutral plane is displaced towards the paper contacting face of the fabric in this case.
Dryer fabrics are typically joined end to end to make them endless by a spiral seam, wherein a flattened helical coil is woven or stitched into each fabric end, to extend across the width of the fabric. To join the ends, the loops of the two spirals are interdigitated and a pintle wire inserted along the tunnel thus formed. Examples of such seams are described in PCT/GB 95/02007 or co-pending Application No. 9703297.3.
Ideally, the seam will lie in the plane of the fabric, to minimise any tendency to lie proud to either side. In fabrics with a symmetrical structure (and thus a centrally disposed neutral plane) the seam will be symmetrically disposed. However, with an asymmetric structure such as suggested above wherein the neutral plane is displaced towards the paper contacting surface of the fabric, the spiral seam is displaced to tend to lie in the neutral plane. As a result the spiral seam also tends to stand proud. on the paper contacting side of the fabric face. This results in marking of the paper web and accelerated seam wear which shortens the useful life of the fabric due to earlier seam failure-or replacement when excessive wear is detected.
An object of this invention is to provide a seam for a woven dryer fabric which is arranged to be aligned below-the neutral plane, so that the seam does not stand proud of either face of the belt and marking and accelerated wear are thus avoided.
An object of the invention is to further provide a woven dryer fabric which incorporates such a seam.
According to the invention a seam construction for a woven dryer fabric comprises at each end of the fabric, a helical spiral or an array of loops, stitched or woven into the fabric by means of loop engaging yarns extending in the machine direction of the fabric, in which the neutral plane of the fabric is displaced from the central plane of the fabric towards the paper-contacting side, characterised in that, at each said end, adjacent the respective loops, at least one pair of cross-machine direction yarns are disposed, the yarns of said pair being substantially equal in diameter.
Preferably the fabric comprises at least two layers of cross-machine direction yarns, the yarns in the layer towards the paper-contacting side being lesser in diameter than at least one other of said layers.
There may of course be more than two such pairs of substantially equal yarns adjacent the loops, to establish a narrow strip of fabric which will enable the seam loops to lie in the geometric plane of the fabric rather than in the neutral plane.
Preferably the diameter of the yarns adjacent to the loops is less than the largest yarn diameter in the fabric, and greater than the smallest yarn diameter in the fabric.
There may be more than two layers of cross-machine direction yarns forming the fabric. These may comprise yarns of varying thickness towards the paper contacting face of the fabric.
In an alternative, the pair of loop adjacent yarns may be replaced by a coarse yarn, the diameter of which is greater than the largest yarn diameter in the fabric but less than the sum of yarn diameter in the yarn pair next to the loopxe2x80x94adjacent yarn.
The spiral used to form the seam may have a circular, flattened or other profile, preferably a rectangular cross section which helps to reduce the proudness of the spiral with respect to the paper contacting side of the fabric.
The invention has made it possible to have a raised neutral line fabric with a seam below the neutral line.